Single phase alternating current electric motors are widely used for a variety of different purposes and range in size from very small fractional horsepower motors on up to multpile horsepower sizes. Single phase motors are particularly popular since most home and business alternating current supplies are in the form of single phase power supplies.
Single phase electric motors include a stator core, which is wound with start windings and run windings connected to the source of operating power. These stator windings surround and are inductively coupled to a rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors or multiple winding high and low resistance rotors. All of these configurations, along with various stator winding arrangements, are well known in the electric motor industry.
Typically, the start winding is made of relatively small diameter wire and the run winding is made of relatively large diameter wire, compared to the diameter of the start winding. These windings are physically and electrically angularly displaced from one another on the stator.
In conventional capacitor-start and capacitor-start/capacitor-run motors, a starting capacitor is connected in series with the starting winding and a switch. At motor start-up the switch is closed and the capacitor, in conjunction with the relatively small diameter starting winding, produces a leading current in the starting winding which is approximately equal to and approximately 90.degree. displaced in phase from the lagging current in the main or run winding of the motor. Such arrangements produce high values of starting torque.
Usually, the switch in a conventional capacitor start motor is a centrifugal or thermal switch connected in series with the capacitor and start winding across the input terminals. The run winding then is connected in parallel with this series-connected starting circuit. In such capacitor start motors, the starting condition is such that the instantaneous locked rotor current is high, and the motor starting current demand factor also is high. As a consequence, such motors undergo relatively high operating temperatures and require some type of switch for disconnecting or opening the starting winding circuit after a preestablished rotational speed of the rotor is reached. Because the starting winding of such motors generally is a relatively small diameter wire, overheating can and frequently does occur. Such overheating results in a relatively limited life of the starting winding due to burnout, particularly under overload conditions of operation of the motor.
Applicant's above mentioned co-pending applications and the above mentioned Patent all are directed to capacitor-start/capacitor-run motors which do not use small diameter starting windings, but instead, utilize two series-connected windings (of substantially the same diameter heavy wire) electrically phase displaced 90.degree. from one another on the stator core. One of these windings has a capacitor connected in parallel with it to form a parallel resonant circuit at the operating frequency of the motor. The motors of the above applications and Patent all are high efficiency motors which overcome most of the disadvantages of the prior art capacitor-start/capacitor-run motors.
For the motor of co-pending application '935, however, the starting torque is relatively low. Thus, motors of the type disclosed in application '935 primarily are suitable for use in situations which do not require very high starting torques, such as pumps, blowers, machines tools and many commercial and domestic appliances. For utilization in situations where higher starting torques are required, the motors of co-pending application '241 and the above U.S. Pat. No. 4,674,565 are employed. These motors also use a parallel resonant circuit at the operating frequency of the motor, where the two windings of the motor are connected in series with one another, and one of the windings has a capacitor across it to form a parallel resonant circuit at the operating frequency of the motor. In addition, a second capacitor is connected in series with a switch in parallel with the first capacitor. This switch is closed during start up of the motor and is opened during normal load conditions of operation of the motor. This permits a substantial increase in the starting torque of the motor, but during normal operating or running conditions of the motor, the parallel resonant circuit functions in the same manner as disclosed in the motor of copending application '935.
Applicant also has three other patents directed to single phase motors of the capacitor start type directed to starting control circuits which produce high starting torque. These Patents are U.S. Pat. Nos. 3,036,255; 3,573,579; and 3,916,274. The '255 Patent is directed to a capacitor motor using a centrifugal or relay operated switch in the starting circuit to open the capacitor starting circuit, disconnecting it and the start winding from the motor operation during normal load conditions of operation of the motor.
U.S. Pat. Nos. '579 and '274 both are directed to solid state motor starting control circuits which do not employ mechanical switches. As a consequence, arcing, which is associated with mechanical switches, and other inherent shortcomings of mechanical switches, such as centrifugal switches, are overcome by the solid state circuitry used in the starting control circuit of these two patents. These patents, like other prior capacitor start motors, however, have starting capacitors connected in series with a start winding and the switch; so that starting current is applied through the start winding only during the start up portion of operation and the motor. Once the motor reaches or nears operating running speed, the solid state switch creates an open circuit condition in the starting circuit; and the starting winding is removed from further operation. Consequently, such a solid state motor starting control circuit functions in a manner similar to the mechanical switch circuits of the prior art to control the connection and disconnection of the starting winding from the power input terminals, in accordance with the particular state of operation of the motor.
It is desirable to provide an efficient motor which is capable of producing a high starting torque while still retaining the advantages of the above-identified co-pending applications.